1. Field of the Invention
The present invention relates to a method for performing a handoff in a fast and secure wireless network, and more particularly to a method for minimizing handoff latencies.
2. Description of the Related Art
Conventionally, a local area network (LAN) is a collection of personal terminals, main frames and workstations coupled to a communication link within a distance of 300 meters or less. The LAN is a high-speed communication network for allowing employees in a company to be aware of information, i.e., a distance in which an electric current or radio wave signal can be correctly transferred between the personal terminals, to commonly and most effectively use equipment installed in the company's building. As LANs, wired networks for directly transferring an electrical signal through the communication link have been initially used. The trend has been to replace the wired networks with wireless networks for transferring a signal using a radio wave in accordance with the development of wireless protocols. LANs based on these wireless networks are referred to as wireless local area networks (WLANs). WLANs are based on Institute of Electrical and Electronics Engineers (IEEE) 802.11. IEEE 802.11-based WLANs have seen immense growth in the last few years. It is predicted that the IEEE 802.11-based WLANs will be rapidly developed in the future because of an advantageous effect of convenient network connectivity.
IEEE 802.11 allows for two operating modes, i.e., an ad hoc mode and an infrastructure mode, in relation to a media access control (MAC) layer. In the ad hoc mode, two or more wireless stations (STAs) recognize each other and establish a peer-to-peer communication without any existing infrastructure. Meanwhile, in the infrastructure mode, there is a fixed entity called an access point (AP) that bridges all data between the STAs associated with the AP. The AP and the STAs associated with the AP form a basic service set (BSS) communicating on the unlicensed radio frequency (RF) spectrum.
FIG. 1 is a view illustrating the architecture of a conventional wireless local area network (WLAN) for supporting the infrastructure mode.
Referring to FIG. 1, a plurality of access points (APs) 120a and 120b are connected through one distribution system (DS) 110. The DS 110 is implemented with a wired network. A communication path is formed between the plurality of APs 120a and 120b. The plurality of APs 120a and 120b form constant service areas, and serve as bridges between STAs 130a, 130b, 130c and 130d and the DS 110. One AP and the STAs associated with the AP form a basic service set (BSS). In other words, a unique BSS is formed on an AP-by-AP basis, and service is provided on a BSS-by-BSS basis. A plurality of BSSs formed by the APs 120a and 120b can be extended to extended service sets (ESSs). The STAs 130a, 130b, 130c and 130d must undergo an authentication procedure to access the WLAN through the APs 120a and 120b to which the STAs 130a, 130b, 130c and 130d belong. In other words, the STAs 130a, 130b, 130c and 130d are permitted to access the network through the authentication procedure. There is provided state information required so that the STAs 130a, 130b, 130c and 130d can access the network according to the authentication procedure. The state information contains encryption information (based on an encryption code) used to transfer data to the DS 110.
In the WLAN based on the architecture shown in FIG. 1, a wireless station (STA) has mobility and hence can move from one BSS to another BSS. In this case, a handoff is required so that service being received from the one BSS can be continuously provided to the STA by another BSS. An AP to which the STA had physical layer connectivity prior to the handoff is referred to as a “prior-AP”, while a new AP to which the STA acquires physical layer connectivity after the handoff is referred to as a “post-AP”.
The conventional handoff procedure refers to the mechanism or sequence of messages exchanged between the APs and the STA. In the conventional handoff procedure, physical layer connectivity and state information must be transferred from one AP to another AP with respect to the STA in consideration. The handoff is a physical layer function carried out by at least three participating entities, i.e., an STA, a prior-AP and a post-AP. The state information that is transferred typically consists of the client credentials (which allow the STA to gain network access) and some accounting information. An operation for transferring the state information can be performed by an inter access point protocol (IAPP). For an IEEE 802.11 network that has no access control mechanism, there would be a nominal difference between a completion association and a handoff/reassociation. Looking at it another way, handoff latency would be strictly greater than association latency as there is an additional inter-access point communication delay involved.
Logical steps based on the handoff procedure are classified into a discovery phase and a reauthentication phase.
1. Discovery Phase: Attributing to mobility, the signal strength and the signal-to-noise ratio of a signal from the STA's current AP (or prior-AP) might degrade and cause it to initiate a handoff. At this point, the STA might not be able to communicate with its current AP (or prior-AP). Thus, the STA needs to find potential APs in range to associate to. This is accomplished by a MAC layer function (or scan function). During a scan, the STA listens for beacon messages sent out periodically by APs at a rate of 10 ms, on assigned channels. Thus, the STA can create a priority list, i.e., a list of APs prioritized by the received signal strength. Two kinds of scanning methods defined in the standard are based on an active mode and a passive mode. As the names suggest, in the active mode, apart from listening to beacon messages (which is passive), the STA sends additional probe broadcast packets on each channel and receives responses from APs. Thus, the STA actively searches or probes for potential APs.
2. Reauthentication Phase: The STA sends a reauthentication request to potential APs according to the priority list in the above-described discovery phase. The reauthentication phase typically involves an authentication and a reassociation to the post-AP. The reauthentication phase involves the transfer of credentials and other state information from the prior-AP. As mentioned earlier, this can be achieved through a protocol such as the IAPP. The reauthentication phase includes an authentication phase and a reassociation phase.
FIG. 2 is a view illustrating a handoff procedure in the conventional WLAN. It is assumed in FIG. 2 that the discovery phase is performed in the active mode. The handoff procedure shown in FIG. 2 is divided into a probe phase 210 and a reassociation phase 220.
Referring to FIG. 2, a wireless station (STA) sensing the need for the handoff transmits a probe request message to a plurality of unspecified APs at step 212. The probe request message is defined as information for asking each AP whether or not the handoff can be successfully performed. Upon receiving the probe request message, the APs transmit probe response messages to the STA at step 214. Here, the fact that certain APs have received the probe request message means that the APs are adjacent to the STA. Thus, the APs capable of receiving the probe request message are determined to be potential APs. The STA repeatedly performs the above-described operation on a channel-by-channel basis.
On the other hand, the STA performs the reassociation phase 220 according to priorities of the potential APs registered in a priority list created in the discovery phase. The STA transmits a reassociation request message to a new AP at step 222. In response to the reassociation request message, the new AP performs an inter access point protocol (IAPP) procedure with other APs, i.e., a prior AP of the STA, at step 230. Through the IAPP procedure, the new AP receives credentials and other state information assigned to the STA. Then, the new AP transmits, to the STA, a reassociation response message to the reassociation request message at step 224.
As described above, the conventional handoff procedure starts when the STA transmits a probe request message and ends when the STA receives a reassociation response message. During the handoff procedure, three types of delay are incurred as in the following. The three types of delay include a probe delay incurred in the discovery phase, an authentication delay incurred in the authentication phase and a reassociation delay incurred in the reassociation phase.
1. Probe Delay: Messages transmitted for an active scan at the probe phase 210 shown in FIG. 2 are probe messages. The latency for this process is referred to as a probe delay. The STA transmits a probe request message and waits for responses from APs on each channel. The time during which the STA waits on a particular channel after sending the probe request message corresponds to probe-wait latency. This is determined to be a time difference between subsequent probe request messages. Here, the time is subsequent between PROBE REQUEST MESSAGES on differing channels. According to the above procedure, it has been found that the traffic on the channel and the timing of probe response messages affect the probe-wait time.
2. Authentication Delay: This is the latency (not shown in FIG. 2) incurred during which authentication frames are exchanged. Authentication consists of two or four consecutive frames depending on the authentication method used by the AP. Some wireless network interface cards (NICs) try to initiate a reassociation prior to the authentication, which causes an additional delay in the handoff process.
3. Reassociation Delay: This is the latency incurred during which reassociation frames are exchanged in the reassociation phase 220 shown in FIG. 2. If an authentication process is successful, the STA sends a reassociation request frame to the AP, receives a reassociation response frame, and completes the handoff. Where the IAPP procedure between a new AP and other APs is additionally required, the reassociation delay will further increase.
According to the above, messages during the probe delay form the discovery phase, while the authentication and reassociation delays form the reauthentication phase. Apart from the latencies discussed above, there will potentially be a bridging delay caused by the time taken for the MAC address updates to Ethernet switches which form the distribution system (i.e., the backbone Ethernet). It can be seen that many latencies are incurred while a handoff between an STA and APs is performed in the conventional WLAN. There are problems in that the latencies not only affect the quality of service (QoS) but also disable high-speed roaming.